OT - Image tranparency the wrong color in SDL app

In my project, all my sprites have been downloaded from the same website. I have a black background in my SDL window. All the sprites have transparent backgrounds. One of them shows up on a white square, but when I open that sprite file in GIMP it has a transparent background. How can I find out how to fix this? I can't even figure out how to form it into a single sentence for a google search...